1. Field of the Invention
The invention is in the field of integrated optical devices. More particularly it relates to an optical switching device for switching a light signal.
2. Prior Art
In a coherent optical local network, a subscriber is connected by means of a combined coherent optical transmitter/receiver. Such a combined transmitter/receiver is preferably constructed as an integrated device. In such a transmitter/receiver, a switch should be incorporated for closing the transmission channel in order to be able to adjust the transmitter without disrupting the network.
For such a switch, an isolation or extinction ratio is required of at least approximately 40 dB. Various types of switches, 1.times.2 or 2.times.2, are known, such as the directional coupler, the digital optical switch, or a Mach-Zehnder switch. These known switches generally have an isolation of 20 to 30 dB and do not therefore achieve the required isolation. It is true that the required isolation can be achieved by placing two or more such switches in series. This means, however, that the complexity and the integration space needed increases. Another switching possibility, which directly fits in with the use of semiconductor material, can be obtained by means of charge carrier injection. As a result of injecting free charge carriers into the semiconductor material over a certain length of a waveguide via an electrode, the absorption increases. In principle, any desired isolation could be obtained simply by making said length and/or the current of charge carriers large enough. An absorption switch of this type would, in addition, be very simple since it can be formed by a waveguide and an electrode placed lengthwise thereon. With the present state of the art of making contact, however, semiconductor materials, such as InP, do not withstand current densities greater than 20 kA/cm.sup.2, which is equivalent to a length of not less than 1 cm for the desired 40 dB isolation. In view of the fact that the typical dimensions of a `chip` of semiconductor material are at present between 1 and 2 cm, this is long.
Reference [1] discloses a directional coupler on semiconductor material, which directional coupler makes use, for the control thereof, of charge carrier injection over the length of the coupling section with the aid of an electrode placed centrally above the central coupling section. In this switch, too, the isolation or extension ratio is of the order of 20 dB.
In a bimodal waveguide, light signals can generally propagate, within a particular wavelength range, both in a zeroth-order guided mode and in a first-order guided mode. If said bimodal waveguide merges into a monomodal waveguide via a taper, only the zeroth-order mode component of the signal in the bimodal waveguide propagates further in the latter and the first-order guided mode component is scattered in the taper. If the bimodal waveguide debouches in an asymmetrical Y junction having a bimodal input guide and two monomodal output guides with different propagation constants, the two mode components are split, specifically in such a way that the zeroth-order guided mode component will couple out via the output guide having the highest propagation constant and the first-order guided mode component via the one having the lowest propagation constant and will propagate further therein in the zeroth-order mode of said guide. In both cases, the two signal components are therefore separated from one another in this process, one of the components being lost, however, in one case. Reference [2], which had not been laid open for inspection to the public in time, describes a passive integrated optical device which can be dimensioned in such a way that a well-defined signal fraction, up to 100%, of a zeroth-order guided mode is converted therewith in a wavelength-selective manner into a first-order guided mode in a bimodal waveguide. If such a converter is coupled at its output to a taper or an asymmetrical Y junction as indicated above and if the conversion of the signal fraction were also to be switchable between two states in which conversion does or does not take place, respectively, a switch would be produced for switching the signal fraction. If the taper is chosen as output section, an on/off switch is produced, while an asymmetrical Y junction produces a propagation direction switch. In view of the constituent components of such a switch, it may be expected that it can overcome the drawbacks mentioned above. In addition, a wavelength-selective multiplexer and demultiplexer described in reference [3], which had not been laid open for inspection to the public in time, could be of switchable construction. There is therefore a need for an optical device of a type similar to the mode converter which is described perse in reference [2] and in which the conversion is switchable.